As a method of processing sapphire used as a substrate material when LEDs or electronic devices are manufactured, the following method is generally used. An exposing process of a crystal plane of grown sapphire crystal, an outer circumference grinding process, an orientation flat grinding process, and a cutting process are performed and then a cut piece is ground to a predetermined thickness obtained by adding a polishing allowance to a target final thickness of a substrate by using a surface grinding apparatus. After that, a polishing process is performed on the surface-ground sapphire substrate. The sapphire substrate has different properties depending on plane orientation thereof, and it is possible to select a substrate having more suitable plane orientation according to an application thereof. For example, it is known that a polar plane such as a C-plane or a semi-polar plane such as an R-plane is suitable for an application as a crystal growth substrate material such as materials of LEDs or electronic devices, and a non-polar plane such as an A-plane or an M-plane is suitable for an application as a substrate for thin film or wind protection of a timepiece in which a flawless substrate with a high hardness is required. However, sapphire has a very stable chemical property, that is, the sapphire is extremely difficult to be affected by acid or alkali, and has the second-highest hardness behind diamond. For this reason, in a polishing process, a sapphire substrate is finished with a desired surface roughness or flatness by using a high hardness material such as diamond as an abrasive grain. However, since damage caused by processing exists on the surface of the sapphire substrate which has been processed by the cutting or grinding process described above and there is a need for removing a surface layer having such damage in order to obtain a smooth surface, significant processing time is required for polishing of the sapphire substrate. From such a point of view, regarding a polishing composition to be used in an application to polish a sapphire substrate, it is important to achieve a decrease in abrasive grain cost and a high polishing rate.
JP 2008-531319 W (US 2006/196849 A) discloses a method of polishing a surface (for example, a C-plane or a R-plane) of a sapphire substrate with a polishing slurry that contains a predetermined amount of an inorganic abrasive material such as colloidal silica dissolved in an aqueous medium containing a sufficient amount of a salt compound dissolved therein as an additive and has a basic pH. According to this, a rate of sapphire removal is increased by about 45% as compared with a polishing slurry that does not contain the salt compound.
Further, JP 2008-44078 A discloses a polishing composition containing a relatively high concentration of colloidal silica which is used to make a surface of a sapphire substrate smoother, and a polishing method using the same.